Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies are used to investigate the problems related to GaAs/InGaP/GaAs heterointerfaces grown by metal organic vapour-phase epitaxy (MOVPE). Normal PL features corresponding to the band gaps of GaAs and InGaP are seen for InGaP layer grown on GaAs. However, we observe an intense long-wavelength PL feature if we grow GaAs on InGaP while the features of InGaP and GaAs are suppressed. This PL feature varies significantly in wavelength with changes in the switching sequence and it could not be suppressed by the growth of a thin layer of GaP between InGaP and GaAs. Similar PL features are also observed for interrupted growth of InGaP on GaAs if we introduce AsH3 during the growth interruption. In this case, PLE measurements show that the corresponding PL features arise from an interfacial layer which is created during the interruption and lies sandwiched between the two halves of the InGaP layer. Thus, we show that a deleterious effect arises with the exposure of InGaP surface to AsH3 during the growth process with or without the Ga source and is attributed to the formation of an interfacial InGaAsP layer.